The present invention relates to an apparatus and method for filtering a beam of light and more particularly to an improved light transmission filter suitable for use in an optical analog sensor for determining the position of a movable object in a prescribed path.
Several different types of variable light transmission filters have been proposed. One type of filter uses a thin reflective film sputtered on a transmissive substrate. The sputtered film has a variable thickness for attenuating a beam of light shining through the substrate as a function of the thickness of the film. In practice, the thickness of the film is difficult to control in order to achieve the desired linear accuracy, and, as a result, commercially available filters tend to have linearity errors approximating five percent.
Another known type of variable light transmission filter uses a wedge of glass having an inherent light absorbing property. The longer the path of light through the wedge, the greater the light is attenuated. This type filter requires that the surface of the glass wedge be ground to a particular shape to achieve a linear attenuation. For small ranges of light wavelengths, this glass wedge type device approaches linearity in transmission, but for many purposes the range of wavelengths encountered will be large, thereby destroying the linear characteristics.
A third type of variable light transmission filter utilizes a mask for variably reducing the size of an aperture to attenuate the transmission of light therethrough. Typically, a wedge-shaped mask is used to vary the size of a slit-shaped sensing aperture. This type of filter produces a highly linear attenuation of light provided there is uniform illumination across the slit. However, the center of a given aperture tends to be illuminated more than the edges. This difference in illumination reduces the linearity of attenuation, particularly when the transmitted light is of low density.
With thin film and glass wedge type filters where light is transmitted through media of varying thicknesses, it is difficult not only to obtain an accurate linear light transmission characteristic but also difficult to produce filters that vary according to trigonometric, exponential, logarithmic, or other selected functions.
An object of the present invention is to provide an improved light transmission filter that overcomes disadvantages of the type presently known.
Another object of the present invention is to provide an improved light transmission filter that provides linear transmission characteristics for a beam of light having a large range of wavelengths.
Another object is to provide an improved light transmission filter that is relatively simple in construction, inexpensive to manufacture, and can be manufactured so that each filter has virtually identical properties.
A further object is to provide an improved light transmission filter that is capable of providing a light transmission characteristic that varies in accordance with a selected algebraic function.
A still further object is to provide an improved optical analog sensor for determining the position of a movable member along a prescribed path.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention will be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.